Aaron Peat

Gather.town id
CRD01
Poster Title
Solar prominence diagnostics from non-LTE modelling of Mgii h&k line profiles
Institution
The University of Glasgow
Abstract (short summary)
We investigate a new method to obtain the plasma parameters of solar prominences observed in the Mgii h&k spectral lines by comparing line profiles from the IRIS satellite to a bank of profiles computed with a one-dimensional non-LTE radiative transfer code. Using a grid of 1007 one dimensional non-LTE radiative transfer models, some including a prominence-corona transition region (PCTR), we carry out a novel method to match computed spectra to observed line profiles while accounting for line core shifts not present in the models. The prominence observations were carried out by the IRIS satellite on 19th April 2018.

The models are able to recover satisfactory matches in areas of the prominence where single line profiles are observed. Large values of ionisation degree are found by the procedure. These are found in areas where the line of sight crosses mostly plasma from the PCTR, correlating with high mean temperatures and correspondingly no Hα emission.

The models were unable to recover satisfactory fits in the regions where we see Hα emission. This is due to the complex line shapes manifesting from many unresolved independently moving threads. This issue might be solved in future by increasing the microturbulent velocities in the models to simulate these unresolved movements.
Plain text (extended) Summary
Slide 1 of 4: The observations of the prominence were carried out on the 19th April 2018. This was a coordinated observation by IRIS, Hinode, MSDP, and other ground based observatories. Naturally, AIA observations were also available.
Figures of 171Å, 304Å, and 2796Å show the morphology of the prominence. A single barb is visible in 171Å as well as faint emission from the PCTR. 304Å shows the cooler material in the core of the prominence. 2796Å shows a similar morphology to 304Å, but the limb appears lower.
XRT Al poly/Open observations clearly show the coronal cavity in which the prominence sits. Over the course of the observation, this filter does not show any appreciable changes.
Some of the IRIS MgII h&k spectra obtained contain complex profiles. With one notable example at 120 arcseconds in raster one at slit position 7.
The prominence contains roughly 25% reversed profiles, 25% complex profiles, and 50% single peaked profiles.

Slide 2 of 4: Here we present integrated intensity, line core shift/doppler velocity, full width half max, and asymmetry maps. These were found via the quantile method. This involves calculating the cumulative distribution function (CDF) of the intensity of the individual line profiles over some wavelength range. The wavelength at which the 50% level of the CDF is found (λ50) is defined as the line core. Including the 12% and 88% level (λ12 and λ88) we can calculate the aforementioned quantities.

Slide 3 of 4: Using a one-dimensional non local thermal equilibrium radiative transfer code, PROM, we generated 1007 MgII model profiles, 252 of which are isothermal and isobaric, where the remaining 755 include a PCTR. Originally, there were 1008
models; however, one model did not converge.
To match these line profiles directly with IRIS observations, we must first degrade the line profiles to match the spectral resolution of IRIS. We also employ sub-pixel interpolation.
The synthesized profiles are formed at exactly the rest wavelength of the line(s). This is rarely (if at all) seen in reality. To account for this, we “roll” the synthesized line profiles through some window centred on the rest wavelength of the lines, measuring the RMS at every position. This allows us to find the “best fitting” line, independent of Doppler velocity. As we measure RMS, we also have a statistic to determines how well a synthesized profile fits the observations.
We are also able to recover the Doppler velocity via this method.

Slide 4 of 4: In total, 49% of pixels were found to have satisfactory fits.
Mean pressure appears to remain stable during the observation, fluctuating on average between 0.18 and 0.26 dyne per square centimetre.
The mean temperature also appears stable during the observation, with the mean temperature staying on average between 7800 Kelvin and 11500 Kelvin.
Areas where we recover satisfactory fits correlate with areas of non-zero gamma. Therefore the inclusion of a PCTR in these models better represent the structure of the prominence.
Past studies show that the ionisation degree (number density of HII divided by the number density of HI) is within 0 to 10. However, these past studies did not consider temperatures above 15000K. Above this, the ionisation degree increases exponentially. The higher temperatures recovered here lead to a higher ionisation degree.
Areas where we recover satisfactory fits are correlated with areas of non-zero gamma. This shows that the inclusion of a PCTR in these models better represent the structure of a prominence.
The Doppler maps recovered by the quantile and rolling RMS method appear consistent with one another, showing that the “rolling” aspect of the procedure is working as intended.
URL
a.peat.1@research.gla.ac.uk